During the last 30 years, quantum light has emerged as a new tool for atomic and molecular spectroscopy. One kind of quantum light is the entangled photons pairs (EP) produced by spontaneous parametric down-conversion (SPDC). EP pairs exhibit strong nonclassical frequency and time correlations simultaneously, which allow them to perform non-linear spectroscopy in new ways compared to classical means. For instance, EP has been used as the excitation source of two-photon absorption (ETPA). In ETPA,EP (in time and frequency domain) are absorbed simultaneously by a chromophore. It was theoretically found and later experimentally demonstrated that, due to photon entanglement, the absorbed photon rate depends linearly on the input photon flux. In contrast, with classical light, the dependence is quadratic for TPA. We used this approach in organic and biological molecular systems. For instance, we have demonstrated that endogenous flavins in protein environment can be studied at extremely low excitation light intensity. Flavoproteins are involved in several relevant biochemistry processes. Free and protein-embedded flavins were studied in aqueous solutions. Our results illustrated that flavins are highly sensitive to ETPA, showing a large response compared with classical light-induced TPA.<sup>1</sup> We have used ETPA as an analytical technique. With a CW pumped ETPA experiment we have studied organic and biomolecules (e.g., flavoproteins and chlorophylls) and materials with the potential to perform photonic to electronic spin transduction in molecules. <sup>1-3</sup> In summary, our ETPA experiments can be used to analyze a wide variety of chromophores of chemical and biological significance.

On the use of quantum light for studying molecular spectroscopy: An experimental approach

ACS Spring 2022

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